Partial melting and recrystallization of Archeaan ... - Springer Link

Contrib Mineral Petrol (1990) 105:704-714

Contributions

to

Mineralogy and Petrology 9 Springer-Verlag1990

Partial melting and recrystallization of Archaean komatiites by residual heat from rapidly accumulated flows Martin J. Gole *, Stephen J. Barnes, and Robin E.T. Hill C.S.I.R.O. Divisionof Exploration Geoscience,Private Bag, Wembley6014, WesternAustralia Received January 9, 1988 / Accepted June 20, 1990

Abstract. Spinifex-textured komatiites at Honeymoon Well, Western Australia, show evidence of partial melting and recrystallization of original igneous textures. Their textures and mineral compositions differ markedly from those typical of komatiites. Spinifex olivine plates are bent and broken, while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene, orthopyroxene, minor olivine and plagioclase. Similar granular pyroxene - plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and, in places, form the matrix to a breccia containing corroded fragments of spinifex rock. Thermometry based on the two pyroxene assemblages yields temperatures of 1055~ to 1141 ~ C, just below the low-pressure komatiite solidus. Mineral compositions are different from those of typical komatiites: clinopyroxenes are Al-poor and Cr-rich, olivines are unusually iron-rich and depleted in Cr and Ca, and the low-Ca pyroxene is bronzite rather than the more typical pigeonite. We interpret these observations as the results of thermal metamorphism, partial remelting and subsequent slow crystallization of originally normal spinifex-textured komatiite flows. The rocks in question occupy a 40-70 m interval sandwiched between two olivine-rich units: an underlying 90 m-thick olivine adcumulate layer, forming part of the cumulate zone of a basal 160 m-thick flow, and an overlying 1 kmthick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex-textured flows. Thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption. Conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution. Such reheating is largely dependent on the time interval between the emplacement of successive flows. Calculations suggest that at Honeymoon Well the emplacement interval GeochemexAustralia, P.O. Box 281, West Perth, 6005, WesternAustralia Present address:

Offprint requests to:

M.J. Gole

must have been of the order of 10 years or less. Textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the Agnew-Wiluna belt.

Introduction Komatiite flows are typically differentiated into upper spinifex textured " A " zones and lower cumulate textured " B " zones (Arndt et al. 1977). This arises from the interplay of nucleation and crystallization of olivine from low-viscosity, supercooled lavas (Donaldson 1982; Huppert et al. 1984; Arndt 1986; Turner et al. 1986). Turner et al. (1986) point out that the development of spinifex textures in a komatiite flow could be inhibited by the rapid emplacement of new flows on top of it, with a consequent sudden decrease in the rate of upward heat loss. In the case of very rapid eruption of komatiite lavas, the whole pile could behave as a single complex cooling unit, resulting in significant modification of igneous textures. In this paper, we present textural evidence for partial remelting and recrystallization of a sequence of spinifex textured komatiites sandwiched between two thick olivine cumulate layers. We attribute this textural modification to conductive thermal relaxation of the temperature profile within a sequence of rapidly accumulated flows. We use finite-element thermal modelling to estimate the rate of eruption of these flows, and conclude that at least 1000 m of komatiite accumulated within a period of the order of 10 years.

Honeymoon Well geology The komatiite flows in question are located at Honeymoon Well in the northern part of the Agnew-Wilunagreenstonebelt, Yilgarn Block, Western Australia (Fig. 1). Within the Upper Greenstone sequence of this belt (Naldrett and Turner 1977) komatiitcs are interlayered with felsic pyroclastic rocks, basalts and metasedi-

705 ments. The komatiite sequence can be traced continuously for over 100 km and is dominantly composed of spinifex-textured flows, layered olivine orthocumulates and lenticular bodies of coarsegrained olivine adcumulate (Donaldson etal. 1986; Hill etal. 1989). These bodies, including the one at Honeymoon Well, grade into stratigraphically equivalent olivine orthocumulate and spinirex-textured units and are interpreted as products of crystallization in voluminous lava rivers (Hill et al. 1987, 1989; Barnes et al. 1988). The komatiites at Honeymoon Well consist of olivine orthocumulates, mesocumulates and adcumulates together with volumetrically subordinate spinifex textured rocks, and occur as two stratigraphic units separated by an intervening unit of felsic volcanic and metasedimentary rocks. Differentiation in komatiite flows from both units indicates a consistent westerly younging direction. The intervening sedimentary unit is absent in the central part of the map area (Fig. 2), where the two komatiite units are in direct contact. In this central area, the Upper Komatiite Unit consists dominantly of coarse grained olivine adcumulate, which grades laterally to the north and south through mesocumulates to orthocumulates without intervening spinifex textured zones. The Upper Komatiite Unit is capped by an unknown thickness of thin spinifex textured flows, overturned and dipping to the east (Fig. 2). The Lower Komatiite Unit consists of olivine orthocumulates with thin spinifex-textured flows. The rocks described in this paper come from a diamond drill hole, H W D 3 (Fig. 2), located within the Lower Komatiite Unit at a point where it is directly overlain by olivine cumulates of the Upper Unit. The Lower Unit here dips west at 70 to 80 degrees. The area has undergone lowermost greenschist facies regional metamorphism (Donaldson and Bromley 1981). Olivine adcumulate and mesocumulate rocks are now replaced by lizardite-magnetite-stichtite-brucite and minor antigorite-carbonate and talc-carbonate assemblages. Tremolite and chlorite are present in olivine orthocumulates and spinifex-textured rocks. Igneous textures are well preserved.

O

I

O

120~ o o

27

Honeymoon Well - 27o00,S

Mt Keith

Six Mile Well

'..

-28o00'S

Agnew#j

Petrology of the Lower Komatiite Unit

Diamond drill hole HWD3 (Fig. 2) was collared within the Lower Komatiite Unit, a few tens of metres below the base of the olivine-rich cumulates of the Upper Komatiite Unit, and drilled to the east. It intersected four thin flows, 35 m thick in total (flows 2 to 5), and a lower 170 m thick flow (flow 1 ; see Fig. 3). The A zone of each flow contains random and plate olivine spinifex textures (Donaldson 1982). The lower flow grades downward from olivine orthocumulate beneath the spinifex zone through mesocumulate to coarse grained (0.5 to 1 cm) adcumulate, with a basal unit of orthocumulate. Another 30 m of thin flows occurs between the top of the interval sampled by HWD3 and the base of the Upper Komatiite Unit cumulates.

Whole-rock compositions The flows from HWD3 (Table 1, Fig. 3) fall into the general compositional range of other komatiite flows in the Yilgarn Block (Fig. 4). In flows 2 to 5, the anhydrous MgO content varies from about 26 wt.% in the spinifex-textured zones to around 45 wt.% in the most olivine-rich sections of the B zones. On an M g O A 1 2 0 3 - C a O plot (Fig. 4) the olivine mesocnmulates

0

40I km ] ~ --]

Granite, granite gneiss ~

~

Greenstones

7~

Komatiites

~ Proterozoicsediments /x

Nickeldeposits

Fig. 1. Map of the Agnew-Wiluna greenstone belt showing the location of Honeymoon Well

and adcumulates in flow 1 plot along the M g O - A 1 2 0 3 join very close to the MgO apex, reflecting a small modal proportion of chromite and low igneous porosity. This low porosity or high olivine content is also reflected in the M/Si ratios which are close to the olivine value of 2.00 (Table 1, nos. 14-16). Analyses of rocks from the overlying 1 km of the Upper Komatiite unit (Table 1, nos. 1-3) also demonstrate low intercumulus porosites.

Petrography Spinifex-textured rocks from flows I to 5 have textural features not previously described from komatiites and which are absent from the spinifex-textured flows at the

706 195 II ~

M')

o~Oa 160

o

o u

i

..//. c

o o

03

~

120

Spinifex-textured rocks Olivine orthocumulate

~

.o_

~

._~

Olivine

80o

rocks

o

mesocumu bate and adcumulate Felsic volcanic

o o n

40o

o

o

o

o

MgO

o

0--~

Olivine mesocL Olivine r

< ~] Olivine spinife~ ] Undiffer, J~

Faults a lineam~

.~,,,"~ Diamon~ ..~

Facing ( 0 I

Fig. 2. Geologicalplan of the Honeymoon Well area showing the

distribution of komatiite lithologies and location of HWD3

top of the Upper Komatiite Unit. Olivine spinifex plates from A2 zones are bent and crumpled (Fig. 5). Diffuse veins and patches, consisting mostly of fine, granoblastic orthopyroxene, plagioclase and minor clinopyroxene, with grain sizes of about 100 pro, disrupt the spinifex zones and in places form a breccia-like fabric (Fig. 6). A granoblastic pyroxene-plagioclase aggregate also forms the mesostasis between spinifex olivine plates (Fig. 7). Grain boundaries in the mesostasis are smooth and curved with 120~ triple-point junctions typical of high-grade metamorphic rocks. This is in contrast to the typical mesostasis of spinifex-textured komatiites, which consists of feathery dendritic olivine and clinopyroxene in devitrified glass (Donaldson 1982; Nisbet et al. 1987). The cumulate B-zones of the HWD3 rocks consist of serpentinized olivine pseudomorphs in a granular mesostasis of clino- and orthopyroxene, texturally similar to that in the spinifex zones.

AI20 3

o o

I

25

I

35

I

I

45

I

IJ

I

55 0 2 wt % anhydrous

I

I

4

I

I

I

6

I

8

Fig. 3. Lithologicaland geochemicalprofiles down HWD3 In parts of the spinifex zones, olivine forms stubby bladed grains (Fig. 8) similar to the bladed metamorphic olivine described by Evans and Trommsdorff (1974). These stubby blades are much thicker than the igneous spinifex plates that are present in the same rock and appear to have nucleated and grown on the original random and plate spinifex olivine grains. Superimposed on this high-temperature mineralogy are the effects of low-grade regional metamorphism. Chlorite, tremolite, epidote and prehnite partially replace pyroxenes and plagioclase. Chlorite also occurs as an alteration product of M g - A1 spinel. Veinlets of serpentine transgress all of the above minerals. Relic igneous olivine is rare, being preserved only in rare patches in random and plate olivine spinifex zones. In the B zones of the flows it is entirely serpentinized. Orthopyroxene and clinopyroxene are preserved in both the spinifex and B zones, but have also been partially serpentinized in places. Most of the plagioclase has been replaced by chlorite and epidote. Minor chromite, phlogopite, and pyrrhotite are also present.

Mineral compositions Compositions of clinopyroxene-orthopyroxene pairs (Table 2, Fig. 9) were determined using averages of defocussed beam microprobe analyses to eliminate the effects of fine exsolution. The clinopyroxenes are low calcium augites, more magnesian (Fig. 9), lower in A1203 and higher in Cr203 (Fig. 10) than clinopyroxenes in olivine spinifex zones from other localities.

707 Table 1. Chemical analyses of rocks from HWD3 and the overlying central zone of olivine-rich cumulates a

1

2

3

4

5

6

7

8

Hole depth (m) ESH

H W D 16 387.0 700

HWD24 172.30 320

HWD24 250.70 270

HWD3 83.47 193.7

HWD3 87.60 190.1

HWD3 90.05 187.9

HWD3 91.25 186.9

HWD3 96.40 182.4

Rock

Upper Komatiite Unit

Lower Komatiite Unit - thin flow sequence (flows 2-5)

oAC

oAC

oAC

Rspx

Rspx

Rspx

Pspx

oOC

P20~ FeS

41.99 0.01 0.18 0.21 5.32 0.06 51.52 0.40 0.10 0.02 0.06 0.00 0.14

41.92 0.00 0.12 0.15 3.78 0.07 53.01 0.43 0.00 0.13 0.06 0.00 0.33

41.39 0.01 0.24 0.19 6.03 0.08 51.37 0.46 0.00 0.06 0.06 0.00 0.11

50.18 0.14 5.07 0.49 11.53 0.15 26.14 0.21 5.02 0.15 0.19 0.01 0.75

46.37 0.14 5.39 0.44 11.02 0.16 28.68 0.21 5.66 0.16 0.04 0.01 1.71

44.00 0.14 5.72 0.41 11.11 0.18 30.36 0.20 6.44 0.11 0.02 0.01 1.30

46.61 0.12 6.50 0.55 10.73 0.22 25.69 0.14 7.37 0.16 0.23 0.00 1.66

43.40 0.08 2.79 0.42 10.76 0.25 39.41 0.30 1.71 0.10 0.02 0.00 0.75

M/Si Mg number

1.94 94.5

1.97 96.1

1.98 93.8

0.97 80.2

1.13 82.3

1.25 83.0

1.02 81.0

1.57 86.7

9

10

11

12

13

14

15

16

Hole Depth (m) ESH

HWD3 98.65 180.4

HWD3 104.35 175.4

HWD3 117.80 163.6

HWD3 119.40 162.2

HWD3 122.80 159.2

HWD3 160.80 125.9

HWD3 200.0 91.5

HWD3 250.0 47.7

Rock

Thin flow sequence

SiOz TiO2 A1203 Cr203 FeO MnO MgO NiO CaO Na20

K20

Lower thick flow (flow 1)

Rspx

Rspx

oOC

oOC

Pspx

oAC

oAC

oAC

SiO2 TiO2 A1203 CrzO3 FeO MnO MgO NiO CaO Na20 KzO P2Os FeS

45.19 0.11 6.66 0.53 10.05 0.31 28.05 0.21 6.28 0.15 0.10 0.00 2.36

44.82 0,10 5.44 0.48 9.84 0.32 28.84 0.18 6.05 0.14 0.07 0.01 3.71

42.57 0.03 1.74 0.37 9.13 0.07 44.90 0.32 0.00 0.06 0.00 0.00 0.80

43.56 0.05 4.50 0.48 10.19 0.35 33.98 0.24 4.33 0.11 0.09 0.01 2.10

43.95 0.08 5.36 0.60 10.05 0.33 28.27 0.15 7.34 0.14 0.10 0.00 3.63

41.36 0.00 0.24 0.43 9.76 0.04 46.91 0.41 0.00 0.05 0.00 0.00 0.81

42.27 0.02 0.19 0.25 5.38 0.02 50.87 0.29 0.02 0.00 0.01 0.01 0.66

43.56 0.02 0.15 0.26 6.75 0.04 48.42 0.41 0.04 0.04 0.01 0.01 0.30

M/Si Mg n u m b e r

1.12 83.3

1.15 83.9

1.76 89.8

1.37 85.6

1.16 83.4

1.90 89.5

1.91 94.4

1.79 92.7

" Notes: Analyses recalculated to 100% free of H 2 0 and C02. ESH, Estimated stratigraphic height above base (i.e. the eastern contact) in metres, oAC, Olivine adcumulate; Rspx, r a n d o m spinifex; Pspx, plate spinifex; oOC, olivine orthocumulate, Fe calculated as FeO and FeS. M/Si = (FeO + M n O + NiO + MgO)/Si02 in molecular weight, Mg number = MgO/(MgO + FeO) in mole percent

708

MgO ' HWD • l 3rocks

20

"~ t i

* e q

wt %

40/

CaO /

/)/

\. I._

/ olivinespinifextexturedzones

~

?

2AI203 kF~a~iltesand-~9

'~

/ ,on ,,c basa,ts ",,,,

Fig. 4. HWD3 rocks plotted within part of an MgO-A12Oa - C a O diagram. Compositional field of other Yilgarn Block komatiites from Binns et al. (1982) is also shown

Fig. 5. Bent olivine spinifex from the top section of the lower thick flow in HWD3 at an estimated stratigraphic height of 159.2 m. Olivine plates are mostly serpentinized and occur in a matrix of orthopyroxene, clinopyroxene, minor olivine, plagioclase and alteration products. Photograph of polished rock slab

Fig. 6. Breccia-like fabric in fine-random spinifex-textured rock from HWD3 at an estimated stratigraphic height of 188.3 m. Nonspinifex areas (A) are composed of mostly polygonal-textured orthopyroxene, plagioclase (mostly altered) and minor clinopyroxene. The spinifex-textured areas (B) are composed of two pyroxeneolivine-plagioclase assemblages. Olivine has mostly been altered to serpentine and magnetite. Photograph of polished rock slab

Fig. 7. Photomicrograph of recrystallized plate spinifex rock with mostly serpentinized olivine plates in a groundmass composed of granoblastic clinopyroxene, orthopyroxene, minor olivine and plagioclase. Chlorite, tremolite and white mica are also present. Sample from estimated stratigraphic height of 186.9 m

Fig. 8. Stubby-bladed metamorphic olivine (mostly serpentinized) that appears to have nucleated and grown on original random olivine spinifex blades. Matrix between olivine blades consists of polygonal aggregate of pyroxene, minor olivine and plagioclase. Sample from estimated strateigraphic height of 179.2 m The orthopyroxenes are compositionally homogeneous bronzites. Bronzite has not been previously reported f r o m r a n d o m or plate olivine spinifex zones, although it is present in a zone with branching olivine in a flow at Alexo, Ontario (Barnes 1985; Fig. 9). The low-Ca pyroxene in spinifex zones is normally pigeonite (e.g. Arndt and Fleet 1979; Barnes et al. 1983). Olivine compositions f r o m the H W D 3 spinifex-textured rocks are given in Table 3. The composition is homogeneous within individual samples and there is a narrow range in forsterite content f r o m F078.3 to FOso.9 between samples. The low forsterite content and the homogeneity of these olivines contrasts with typical spinifex olivines, which are zoned with cores of F095 to F09o and rims o f F092 to F084 (Donaldson 1982). The C a O and C r 2 0 3 contents are significantly lower, and the M n O contents slightly higher in the H W D 3 olivines c o m p a r e d to other spinifex olivines which typically con-

709 Table 2. Electron microprobe analyses of pyroxenes in spinifex-textured rocks from HWD3 and temperatures calculated from pyroxene thermometers a Depth (m) in HWD3 ESH

83.47 193.7

84.5 192.8

86.2 191.3

86.4 191.1

86.6 190.9

87.5 190.2

cox

opx

cpx

opx

cpx

opx

cpx

opx

cpx

opx

cpx

opx

:53.13 0.36 2.33 0.84 5.08 0.15 17.52 20.75 0.24

55.26 0.21 1.40 0.43 11.70 0.21 29.89 1.51 0.03

53.08 0.39 2.22 0.89 5.51 0.12 16.77 20.58 0.21

55.51 0.21 1.53 0.45 11.87 0.25 28.91 t.53 0.03

53.37 0.35 2.31 0.94 5.18 0.17 16.36 21.19 0.25

55.20 0.20 1.41 0.47 11.90 0.28 28.71 1.52 0.03

53.38 0.31 2.31 0.95 5.34 0.18 16.49 21.24 0.24

55.36 0.19 1.47 0.40 12.28 0.26 28.53 1.45 0.02

53.33 0.41 2.36 0.95 5.22 0.14 16.63 21.04 0.25

55.42 0.20 1.42 0.45 11.70 0.25 28.91 1.25 0.02

53.01 0.56 2.10 0.85 5.49 0.15 16.64 21.31 0.24

55.56 0.28 1.37 0.41 11.02 0.24 29.10 1.69 0.03

100.40

100.64

99.77

100.29

100.12

99.72

100.44

99.96

100.33

99.62

100.35

99.70

Ca Mg Fe Mg/(Mg + Fe)

42.3 49.6 8.1 86.0

2.9 79.6 17.2 82.0

42.7 48.4 8.9 84.4

3.0 78.9 18.1 81.3

44.1 47.4 8.4 84.9

3.0 78.7 18.3 81.1

43.9 47.5 8.6 84.6

2.8 78.3 18.9 80.6

43.6 47.9 8.4 85.0

2.5 79.5 18.0 81.5

43.8 47.8 8.4 85.1

3.3 79.7 16.9 82.5

N

37

23

13

25

14

37

25

37

20

30

13

SiO2 TiO 2 A120 3 Cr2Oa FeO MnO MgO CaO Na20 Total

8

Calculated temperatures ~ D&L W

Sample no. HWD3 m ESH

1118 1093

1130 1107

1088 1055

89.7 188.3

87.6 190.1

1087 1058

90.05 187.9

1102 1068

106.6 173.4

123.4 158.6

cpx

opx

cpx

opx

cpx

opx

cpx

opx

cpx

SiO2 TiOz A1203 Cr203 FeO MnO MgO CaO NaaO

53.23 0.39 2.41 0.86 4.78 0.12 17.67 20.99 0.23

55.04 0.19 1.37 0.37 11.09 0.24 30.03 1.31 0.02

52.93 0.40 2.i0 0.97 4.78 0.13 17.31 21.25 0.24

54.90 0.21 1.46 0.46 11.10 0.21 30.30 1.42 0.02

53.41 0.37 2.13 0.90 4.86 0.16 16.58 21.48 0.25

55.39 0.20 1.38 0.49 11.50 0.23 29.08 1.75 0.03

53.43 0.30 2.45 1.02 4.76 0.16 17.27 20.90 0.21

55.89 0.21 1.48 0.54 10.27 0.22 29.69 1.86 0.03

52.79 0.20 2.69 1.18 4.83 0.18 16.77 21.33 0.22

Total

100.90

99.66

100.11

100.08

100.14

100.05

100.50

100.19

100.19

Ca Mg Fe Mg/(Mg+Fe)

42.4 49.7 7.9 86.3

2.5 80.8 16.7 82.9

43.3 49.1 7.6 86.6

2.7 80.7 16.6 82.9

44.4 47.7 7.8 85.9

3.4 79.0 17.5 81.8

43.0 49.4 7.6 86.6

3.6 80.7 15.6 83.8

44.0 48.2 7.8 86.1

N

29

34

20

35

24

37

22

17

9

1055 1065

Calculated temperature~ D& L W

1125 1104

1087 1083

1064 1050

1141 1103

1120

Notes Analyses done by energy dispersive spectrometry for major elements and wavelength spectrometry for minor elements. ESH, Estimated stratigraphic height in metres above the lower eastern margin of the komatiite sequence; N, number of analyses averaged; D & L, Davidson and Lindsley (1985); W, Wells (1977). For calculations in Davidson and Lindsley (1985), thermometer F e O = a l l Fe in pyroxene compositions and a pressure of 250 bars was used. x in sample HWD3-123-4 all orthopyroxene was altered and a likely composition was assumed for the thermometry calculation a

710 Table 3. Electron microprobe analyses of olivine in spinifex-textured rocks from HWD3 Depth (m) in HWD3 ESH

83.47 193.7

84.5 192.8

86.2 191.3

86.4 191.1

86.6 190.9

87.5 190.2

190.1

89.7 188.3

SiO2 AlzOa CrzO3 FeO MnO NiO MgO CaO

39.18 0.0l 0.00 18.94 0.20 0.22 41.61 0.04

39.34 0.02 0.00 19.40 0.24 0.14 40.99 0.04

38.70 0.02 0.02 18.79 0.28 0.09 42.83 0.04

39.01 0.01 0.00 20.05 0.28 0.11 40.64 0.03

39.36 0.01 0.02 18.97 0.26 0.10 41.39 0.04

39.45 0.01 0.03 17.87 0.24 0.19 42.36 0.04

39.04 0.02 0.02 18.09 0.23 0.08 42.61 0.04

38.85 0.00 0.00 18.15 0.24 0.16 42.70 0.03

Total

100.20

100.17

100.77

100.13

100.15

100.19

100.13

100.13

79.7 13

79.0 11

80.2 14

78.3 17

79.5 19

80.9 6

80.8 10

80.7 16

Fo N

Table 4. Electron microprobe analyses of plagioclase in spinifextextured rocks from HWD3 Depth (m) in HWD3 ESH

83.47 193.7

86.6 190.9

87.5 190.2

87.6 190.1

SiO 2 A1203 FeO MgO CaO K/O Na20

44.31 35.36 0.20 0.02 18.23 0.03 1.18

46.86 34.48 0.27 0.01 16.98 0.01 1.75

46.51 34.64 0.32 0.03 17.45 0.02 1.60

46.15 34.77 0.31 0.00 17.30 0.00 1.53

Total

99.33

100.42

100.57

100.06

An Ab Or

89.4 10.5 0.2

84.2 15.7 0.1

85.6 14.3 0.1

86.2 13.8 0.0

87.6

1.4

1.2

1.0 9

0.8

o

0.6

II

0,4

;

50 ,\ /"

~

;Y" 4O

/

\

9

0.2

.

t~l

X

v

C

x

9 "C) u ~)

mol %

2~ Mg 100

:,.;., o 90

80

7O

Fe

i

I

2

I

I

4 6 AI20 3 wt %

I

8

HWD3 Fleet and MacRae (1975) Nisbet et al (1977) Arndt et al (1977)

10

x Barnesetal(1983) o Lewis and Williams (1973) + Barnes (1985)

Fig. 10. Cr203 and AlzO3 relations of clinopyroxene from HWD3 and Ca-rich clinopyroxene from other olivine spinifex zones. The two A1203-poor pyroxenes from Barnes et al. (1983) have equant morphologies and are from immediately below the A 2 zone, while the single clinopyroxene from Barnes (1985) is from a zone of branching olivine

Fe 9 o (3 +

HWD 3 Lewis and Williams (1973) Fleet and MacRae (1975) Nisbet et al (1977)

9 C, Arndt et al (1977) x Barnes et al (1983) u Barnes (1985)

Fig. 9. HWD3 pyroxenes plotted in part of a C a - M g - F e diagram together with igneous pyroxenes from random and plate olivine spinifex zones at other localities, the pyroxenes from Barnes (1985) are from a zone of branching olivine spinifex

tain 0.1-0.3 wt.% CaO, 0.15-0.3 wt.% Cr203, and up to 0.2 wt.% MnO (Nisbet et al. 1977; Arndt et al. 1977; Smith and Erlank 1982; Barnes et al. 1983). Plagioclase varies in composition from Ans4 to Ans9 and has a very low orthoclase component (Table 4). We are not aware of other published plagioclase analyses

711 from spinifex zones of Archaean komatiites although plagioclase (An56 to An83) is present in the Mesozoic Gorgona Island spinifex rocks (Echeverria/980). Interpretation of the HWD3 rocks

The textures in the HWD3 rocks are in marked contrast to those normally found in olivine spinifex-textured komatiites, where olivine blades are thin and straight and the mesostasis consists of fine skeletal clinopyroxene and altered devitrified glass (Arndt et al. 1977; Donaldson /982; Nisbet et al. /987). In the HWD3 rocks the bent spinifex blades, the orthopyroxene-plagioclase rich veins and the polygonal two-pyroxene, olivine and plagioclase intergrowths overprint or modify igneous textures typical of spinifex-textured rocks. The HWD3 textures are themselves overprinted by low-grade regional metamorphism. The lack of compositional zoning in minerals, the compositions of clinopyroxene and olivine, and the presence of both bronzite and relatively coarse grained plagioclase in the HWD3 spinifex rocks are also distinctly different from other olivine spinifex zones with generally similar bulk rock compositions. The pyroxene assemblage in the HWD3 spinifex rocks is indicative of slow equilibrium crystallization, based on the presence of bronzite and the low A1203 content of clinopyroxene. High AlzO3 contents typical of igneous clinopyroxenes in spinifex zones are attributed to high growth rates and to the lack of competition for AlzO3 from plagioclase (Donaldson/982). The HWD3 pyroxene assemblage apparently grew more slowly, in equilibrium, and in the presence of plagioclase. The unusually iron-rich composition and homogeneity of olivines also suggests slow cooling and equilibrium crystallization, rather than the normal case of rapid crescumulate growth (Barnes 1985). We attribute the unusual textures and mineralogy of the HWD3 rocks to partial remelting and protracted recrystallization of what were originally normal spinifextextured rocks. The irregular veins and patches of granular textured pyroxene-plagioclase rock represent remelting, small-scale remobilization and slow recrystallization and annealing of the original mesostasis. In situ slow crystallization and annealing of the mesostasis also occurred within A3 and B zones, resulting in the granular pyroxene-plagioclase matrix between cumulate and bladed olivine grains (Fig. 7). The bending and crumpling of the spinifex olivine plates (Fig. 5) are key pieces of evidence. These structures are non-penetrative, have no associated foliation and therefore cannot be tectonic in origin. In view of the rarity of these structures elsewhere, and their association with the unusual mineralogy of the HWD3 flows, they are unlikely to have formed during flow and initial crystallization of the flows. They are interpreted as the result of compaction of partially molten A-3 spinifex zone beneath the weight of the overlying komatiite pile. The only similar occurrence of bent spinifex textures of which we are aware is in rocks from the hanging-wall ore zone at Lunnon shoot, Kambalda (Groves et al.

/986). Underground exposures leave no doubt that the Lunnon Shoot rocks have been partially melted by an overlying komatiite flow, and that the mesostasis has been filter-pressed out from between the original spinifex plates. We are confident in extending a similar explanation to the HWD3 rocks. However, whereas the Lunnon Shoot example is restricted to a single flow top, and is attributable to thermal erosion of this flow by the overlying one, partial remelting and recrystallization textures in HWD3 are distributed throughout a stratigraphic interval of 40 m, and are not restricted to flow tops. They must therefore be due to wholesale thermal metamorphism of this part of the komatiite pile.

Two-pyroxene geothermometry Thermometry based on analyses of pyroxene pairs from the HWD3 rocks yields a range of 1055 to/141 ~ C using the method of Davidson and Lindsley (/985), and a slightly lower range using the Wells (/977) geothermometer (Table 2). Calculated temperatures do not vary systematically with depth over an estimated true thickness of 35 m. These temperatures are 30-70~ C below the I atm solidus temperature obtained by Arndt (/976) for a komatiite of similar composition to the HWD3 spinifex-textured rocks (i.e. 25% MgO).

Thermal modelling Geothermometry of the HWD3 rocks indicates that these flows have undergone thermal metamorphism, recrystallization and annealing at temperatures in excess of 1100 ~ C. Numerical modelling of thermal conduction in the volcanic pile provides some constraints on how this may have occurred. If a sequence of lava flows accumulates rapidly, an earlier flow may still be hot when the next flow arrives on top of it. This is particularly relevant where the early flow contains a high proportion of cumulus olivine, such that liquid convection is inhibited and cooling is dominantly by conduction. The centre of a thick extrusive body is likely to remain at a temperature close to the liquidus temperature of the parent lava for a period of tens or hundreds of years, depending on its thickness, while the top of the flow is cooled rapidly to the ambient surface temperature. At a given time during cooling, a sequence of such flows would display a roughly sinus0idal temperature profile, each flow unit having a hot centre and cold margins. As this temperature profile relaxes with time, original flow tops become reheated while the cumulate zones cool. In the right circumstances, chilled flow tops and spinifex zones could be partially remelted or extensively recrystallized by this mechanism. This is most likely to happen in the case where thick flows are erupted in rapid succession. We have modelled this process for a simplified analogue of the Honeymoon Well case. We treat the lower thick flow (flow 1 of HWD3) and the 1-km-thick olivine cumulate unit of the Upper Komatiite Unit as being

712

eruptions: 3, 30 and 100 years. In each case, curves are shown at various time intervals (in years) after emplacement of flow 1. Figure 12 shows corresponding curves of temperature variation of tops and centres of units 1 and 2 as a function of time after eruption of unit 2. Rapid cooling of flow tops and slower cooling of interiors occurs initially after eruption, giving rise to A and B zones in units 1 and 2. Growth of spinifex zones in the upper few metres of the flows corresponds to initial cooling rates of the order of ~~176 in convecting lava (Turner et al. 1986). Once a framework of olivine crystals has formed, within days or weeks of eruption, convection is inhibited and subsequent cooling is by conduction as modelled. In the case of 100-year eruption intervals, significant subsolidus cooling of each flow unit occurs before it is covered by the next one. The tops and centres of both units 1 and 2 are reheated by condution after emplacement of overlying unit 3, achieving maximum temperatures between 700 ~ and 900 ~ C about 500 years later (Fig. 12). As the interval between eruptions decreases, significantly more heat is retained within the pile and higher temperatures are attained. Maximum temperatures around 1000~ C are attained about 100 years after

discrete eruptive units, referred to as flow units 1 and 3, respectively. The intervening thin flows are treated as a single flow 40 m thick, and designated as flow unit 2. The three flow units are assumed to be emplaced instantaneously at equally spaced intervals, and to cool subsequently by conduction. The initial temperature of the mesocumulates and adcumulates is taken to be 1500 ~ C, the assumed liquidus temperature of their parent magma. We interpret the adcumulates and mesocumulates to form by in situ crystallization at the floor of turbulently flowing lava rivers (Hill et al. 1989; Barnes et al. 1988). Consequently, latent heat of initial olivine crystallization of these rocks is advected away in the lava during emplacement and does not enter into the conduction calculations. The effect of latent heat of crystallization of intercumulus liquid within the sequence is neglected, which will result in calculated cooling rates being slightly more rapid than they should be. Thermal profiles as a function of time (Fig. 11) have been calculated using a numerical finite difference method assuming a uniform thermal diffusivity of 0.01 cm2/s (Carslaw and Jaeger 1959). Families of curves are shown for three different values of the time interval between

400

- 30 YEARiNTERVAL

- 100 YEAR INTERVAL

3 YEAR INTERVAL

-6

o 09

.ff

300

// E 200

--.

-• 300/

..c

._~

- \

,/ ~60~

~. 90.t

/Jo

............

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100

40

/

!

200 100

/

'

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/

/gr

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-100

/

0

///

i

500

1000

1500 0

, , 500 1000 Temperature ~

Fig. 11. Calculated temperature profiles as a function of time through the footwall rocks and lower part of komatiite pile consisting of a lower 160-m-thick flow (unit 1), a central 40-m-thick flow (unit 2) and an upper > 500 m thick flow (unit 3), corresponding to Lower and Upper Komatiite units at Honeymoon Well. Three sets of profiles have been calculated assuming emplacement at an

< I

1500 0

500

1000

1500

initial temperature of t 500 ~ C, of the three units at equal intervals of 3, 30, and 100 years. The temperature profiles are shown at periods of years after the emplacement of unit 1. The solidus of a 25% MgO komatiite (Arndt 1976) is also shown. Levels A1, B1, A2 and B2 correspond to curves shown in Fig. 12

713 1500

B2---..-.~

1000 -

A

2

100 YEAR INTERVAL

4 ......

~[

~

L

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co

30 YEAR INTERVAL Solidus

? 1000 2 500

j A I ~

0 1500

soNu~_~' _,.~_B _2

3 YEAR INTERVAL

1000

500

"E

A2 B2 A1 B1

A-zone of B-zone of A-zone of B-zone of

lyr 3yrs 10yrs 30yrs 100yrs Time after emplacement of Unit 2

Unit Unit Unit Unit

2 2 1 1

1000,'rs

but cool at a greatly suppressed rate. The mesostasis of the B-zones therefore crystallizes for the first time as granular pyroxene-plagioclase aggregate. Beyond about 100 years after emplacement of unit 3, independent of the eruption rate, fine-scale thermal perturbations are ironed out, and the whole pile behaves as a single cooling unit. Cooling rates at this stage would be of the order of 1~ C per year. Temperatures recorded by the two-pyroxene assemblages in the HWD3 rocks are probably blocking temperatures set during this phase of the cooling history. This model involves a number of assumptions. In particular, treating the thin HWD3 flows as a single flow is a major simplification, since in practice these flows would probably cool much more rapidly by a combination of convection and conduction as a series of discrete cooling units (Turner et al. 1986). Since these flows only account for about 50 m out of a total of 1200 m thickness of the komatiite pile, the effect of this simplification on the overall thermal budget is small. Taking the uncertainties into account, the calculations show that extensive remelting and recrystallization of the HWD3 flows could have been caused by residual heat and thermal relaxation, as long as the time-scale for emplacement of the successive flow units was of the order of 10 years or less.

Fig. 12. Temperature versus time curves for four positions in the vertical profiles of Fig. 11 for three values of the time increment between eruptions: 3, 30 and 100 years. Curves A1 and A2 correspond to points within the spinifex A-zones of units 1 and 2, while B1 and B2 correspond to points in the centre of cumulus B-zones (see Fig. 11). Note the logarithmic time scale. Time scale is measured from time of eruption of unit 2. Curves B1 and A1 at time zero represent temperatures to which unit 1 has cooled at the time of eruption of unit 2. A2 curve shows initial rapid cooling corresponding to crystallization of spinifex zone. A1, A2 and B2 curves show inflection in temperature trend at time of eruption of unit 3 (vertical line marked " U n i t 3 "), and subsequent reheating during thermal relaxation phase

eruption of unit 3 in the case of a 30-year eruption interval. In the case of a 3-year eruption interval, the centre of flow 1 is still at 1500~ C at the time of emplacement of unit 3, while the upper parts of units 2 and 3 have cooled significantly (Figs. 11, 12). During relaxation of the thermal profile, the A zones of both flows I and 2 are reheated above their solidus temperature about 10 years after emplacement of unit 3, and remain there for several hundred years before cooling at rates of less than 1 degree per year below the solidus (Fig. 12). Maximum temperatures of 1350~ C are attained, which would enable partial or complete remelting of original interspinifex glass and dendritic pyroxene and olivine. The subsequent phase of slow cooling results in crystallization of annealed, granular intergrowths of pyroxene and plagioclase, as observed. Compaction and melt migration also occurs at this stage, resulting in crumpling of spinifex plates and formation of veins and breccias as observed in the HWD3 flows. In the 3-year case, the B-zones of units 1 and 2 show little or no re-heating,

Conclusion The unusual petrographic features of the HWD3 rocks arise from a combination of two factors: the sandwiching of spinifex textured rocks between two thick extrusive units of olivine adcumulate, and the rapidity of eruption of these units. Remelting and recrystallization happened because the upper adcumulate-bearing flow was emplaced before the lower one had cooled down to any great extent. Significantly, remelting and recrystallization textures are absent from spinifex-textured rocks at the top of the Upper Komatiite Unit, where there is no overlying adcumulate unit.

Implications If komatiite flows accumulate so rapidly that the centre of a flow is still close to its emplacement temperature when the next flow arrives, remelting and potentially complete reconstitution of flow tops and spinifex zones may occur. The limiting time scale for this process depends critically on the thickness of individual cooling units. Turner et al. (1986) show that the time taken for an initially phenocryst-free komatiite flow to differentiate into A and B zones is about 1 day for a 1-m-thick flow, 1 month for a 10-m-thick flow and 1 year for a 100-m-thick flow. At this time, the upper part of the flow would be largely solid, while the cumulate B-zone would be at a temperature somewhere between the solidus and liquidus. If successive flows are emplaced within a similar time scale to that of solidification, significant thermal perturbations will be retained in the lava pile,

714 a n d extensive r e m e l t i n g o f flow t o p s a n d spinifex zones m a y result. I f e r u p t i o n is sufficiently r a p i d , successive flows m a y arrive b e f o r e spinifex a n d c u m u l a t e zones o f u n d e r l y i n g flows h a v e h a d time to f o r m a t all. This m a y d e s t r o y the i n t e g r i t y o f i n d i v i d u a l flows, a n d give rise to a l a y e r e d sequence o f h i g h - p o r o s i t y a n d l o w - p o r o sity o r t h o c u m u l a t e s . Sequences o f t h i c k - l a y e r e d o r t h o c u m u l a t e s tens to h u n d r e d s o f m e t r e s thick, w i t h o u t int e r v e n i n g spinifex zones, are a c h a r a c t e r i s t i c f e a t u r e o f the k o m a t i i t e s in the A g n e w - W i l u n a belt (Hill et al. 1987, 1989) a n d m a y owe their origin to these processes. F u t u r e studies o f such sequences m a y p r o v i d e f u r t h e r examples of granular pyroxene - plagioclase textures a n d o t h e r features o f the H W D 3 flows.

Acknowledgements. We thank CRA Exploration and the Minerals and Energy Research Institute of Western Australia for financial support. Paul Ashley and Ray Binns are thanked for their constructive comments on the manuscript.

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ments and applications to geothermometry. Contrib Mineral Petrol 91 : 390404 Donaldson CH (1982) Spinifex-textured komatiites: a review of textures, compositions and layering. In: Arndt NT, Nisbet EG (eds) Komatiites. Allen and Unwin, London, pp 213-244 Donaldson MJ, Bromley GJ (1981) The Honeymoon Well nickel sulfide deposits. Western Australia. Econ Geol 76:1550-1564 Donaldson MJ, Lesher CM, Groves DI, Gresham JJ (1986) Comparison of Archaean dunites and komatiites associated with nickel mineralisation in Western Australia: implications for dunite genesis. Miner Deposita 21:296305 Echeverria LM (1980) Tertiary or Mesozoic komatiites from Gorgona Island, Colombia: field relations and geochemistry. Contrib Mineral Petrol 73:253-266 Evans BE, Trommsdroff V (1974) On elongate olivine of metamorphic origin. Geology 1:131-132 Groves DI, Korkiakoski EA, McNaughton N J, Lesher CM, Cowden A (1986) Thermal erosion by komatiites at Kambalda, Western Australia, and the genesis of nickel ores. Nature 319:136-139 Hill RET, Gole MJ, Barnes SJ (1987) Physical volcanology of komatiites - a field guide to the komatiites between Kalgoorlie and Wiluna, Eastern Goldfields Province, Yilgarn Block, Western Australia (Excursion Guide Book). Geological Society of Australia, Western Australia Division Hill RET, Gole MJ, Barnes SJ (1989) Olivine adcumulates in the Norseman-Wiluna greenstone belt, Western Australia: implications for the volcanology of komatiites. In: Prendergast MD, Jones MJ (eds) Magmatic sulphides the Zimbabwe volume. Institute of Mining Metall, London, pp 189-206 Huppert HE, Sparks RSJ, Arndt NT (1984) Emplacement and cooling of komatiitic lavas. Nature 309:19-22 Naldrett AJ, Turner AR (1977) The geology and petrogenesis of a greenstone belt and related nickel sulphide mineralization at Yakabindie, Western Australia. Precambrian Res 7: 3-30 Nisbet EG, Bickle M J, Martin A (1977) The mafic and ultramafic lavas of the Belingwe greenstone belt, Rhodesia. J Petrol 18 : 521-566 Nisbet EG, Arndt NT, Bickle M J, Cameron WE, Chauvel C, Cheadle M, Hegner E, Kyser TK, Martin A, Renner R, Roedder E (1987) Uniquely fresh 2.7 Ga komatiites from the Belingwe greenstone belt, Zimbabwe Geology 15 : 11421150 Smith HS, Erlank AJ (1982) Geochemistry and petrogenesis of komatiites from the Barberton greenstone belt, South Africa. In: Arndt NT, Nisbet EG (eds) Komatiites. Alien and Unwin, London, pp 347-397 Turner JS, Huppert HE, Sparks RSJ (1986) Komatiites. II. Experimental and theoretical investigations of post-emplacement cooling and crystallization. J Petrol 27:397-437 Wells PRA (1977) Pyroxene thermometry in simple and complex systems. Contrib Mineral Petrol 62:129-139 Editorial responsibility: R. Binns